Since earliest times, materials, objects, and devices have been packaged to reduce contamination during transport or storage. This packaging has utilized a variety of vessels, fabricated from various objects, in an effort to prevent the contamination of the vessels contents from environmental and other sources. Today, packaging is still used for the same purposes. But, the demands placed upon this packaging often exceeds that previously encountered. For example, exposure of many sensitive electronic devices and objects to contaminates is detrimental. In an attempt to control low-level exposure, such devices are packaged in "clean rooms" or in other controlled environments. For many such devices, selection of the packaging materials is as important as the method of packaging. Packaging materials are selected so that contamination caused by the materials of construction of the packaging material is minimized.
Currently, low-level contamination of packaged objects is reduced or eliminated by use of any of four methods. These methods include hermetic sealing, vacuum packaging (which includes hermetic sealing), chemical reaction, and physical adsorption. Hermetic sealing is easily accomplished by package design or by selection of the materials of construction of the package. For example, the common glass jar with lid typically provides a good hermetic seal. This is especially true if the lid is gasketed. Hermetic sealing can also be easily accomplished by use of suitable "plastic" bags equipped with locking seals or heat sealing capability.
Vacuum packaging is also used to prevent low-level contamination. Vacuum packaging involves hermetic sealing along with evacuation of the interior of the packaging. Typically, vacuum packaging requires careful consideration of the packaging design and materials of construction. The package must be designed such that it is possible to apply a hermetic seal while maintaining a suitable interior vacuum. The materials of construction must be selected such that the environmental pressure does not destroy the package's integrity. These materials must also inhibit gas or vapor transport through the package. The materials of construction must also be such that they do not in and of themselves contaminate the packaged object. Therefore, materials that outgas under vacuum are normally avoided. This technique is not as typically employed as hermetic sealing, generally because of the apparatus and procedures, and associated cost, necessary to vacuum package. Accordingly, only those objects most sensitive to the effects of contamination are packaged in evacuated packages due to the complexities and costs involved.
Chemical adsorption of contaminates in packaging is rarely practiced, except in the case of electrical devices. For example, light bulbs are typically supplied with a device referred to as a "getter". This device serves to remove by chemical reaction significant contaminates within the bulb, which may damage the light emitting filament during use, after sealing and prior to final packaging.
The packaging of adsorbents with contents sensitive to contamination is well known. Suitable physical adsorbents include activated carbon or zeolites. For example, small packets of silica gel are commonly placed in packages containing sensitive mechanical devices to lower the humidity within the vessel in order to inhibit the formation of rust or other types of corrosion. Such packets are also introduced into packages containing electronic devices and some food stuffs. The application of this technique is so well known and wide spread that various package sizes of silica gel are sold to consumers for use in containers of their own choosing. Although relatively effective, this technique has certain inherent limitations. For example, the diffusion of water vapor is relatively slow at atmospheric pressure. Therefore, silica gel packets are best distributed throughout the container to provide shorter diffusion paths. Also, as the containers are not typically hermetically sealed, the water capacity of the silica gel packets can be readily exhausted in high humidity environments.
To date, physical adsorption has not been used in combination with vacuum packaging to reduce low-level contamination. Such a combination would be advantageous from the aspect that any contaminate present in the vapor phase within the evacuated packaging would be essentially removed by the adsorbent. It is believed, however, that an inherent characteristic of physical adsorbents limits their usefulness in such applications. It is very difficult to remove adsorbed materials from adsorbents by simple application of vacuum. Therefore, inclusion of adsorbents in packages to be hermetically vacuum sealed would lead to excessively long evacuation times. Also, even evacuated, the adsorbent would continue to desorb gases and vapors unless the evacuation time period was extremely long. This desorption would then compromise the package vacuum.
The ability of adsorbents to adsorb gases, even at low pressures is well known. Activated carbons have been used to store liquefied gases (U.S. Pat. No. 2,760,598) and as a means to maintain vacuum in closed vessels (U.S. Pat. No. 3,921,844). It is also know that cooling the carbon increases it's effectiveness.
Accordingly, it is the object of the present invention to provide a method for vacuum packaging materials, objects, and devices which provides for improved protection from contamination. It is another object of the invention to provide a package or means for hermetically sealing an object at reduced atmosphere. It is a further object of the invention to provide both a method and means for facilitating hermetically packaging objects at reduced atmosphere without the need for out gassing the package prior to sealing.